1. Field of the Invention
The present invention relates to a ceramic honeycomb structural body suitable for a catalyst carrier, wherein a number of flow channels having a polygonal cell-like cross-section and extending longitudinally through the honeycomb structural body are defined by a peripheral wall and partition walls arranged inside of the peripheral wall.
The present invention also relates to a ceramic honeycomb catalyst comprising such a honeycomb structural body and a catalytic substance carried thereon.
2. Description of the Related Art
The ceramic honeycomb structural body having the above-mentioned arrangement is widely used, for example, as a catalyst carrier in an exhaust gas cleaning system for automobiles. Use of ceramic honeycomb structural bodies as catalyst carriers has been wide spread due to low pressure loss during passage of the exhaust gas as a result of an high open frontal area, and an excellent exhaust gas clarifying performance. In this connection, an advanced ceramic honeycomb structural body which has been put into practical use has, for example, a partition wall thickness of 0.170 mm and a cell density of 60 cells per 1 cm.sup.2.
In accordance with a recent enhancement in the exhaust gas regulation as related to environmental problems, e.g., a requirement for reduction in the total emission amount of hydrocarbon in the LA-4 mode which is one of exhaust gas evaluation test modes in the United States, there is a strong demand for an improved ceramic honeycomb structural body which is capable of achieving a distinguished exhaust gas clarifying performance as compared to conventional honeycomb structural bodies. Specifically, in an operational state immediately after starting an engine, i.e., in the so-called cold start state, the exhaust gas clarifying efficiency undergoes a considerable deterioration because the catalyst is still relatively cold and hence it is not sufficiently activated. Thus, an early activation of the catalyst during the cold start state is considered as the most important task to clear the exhaust gas regulation.
From such a viewpoint, as a general discussion, it has been proposed to reduce the thickness of the partition walls of the ceramic honeycomb structural body. The thin-walled ceramic honeycomb structural body serves on one hand to increase the open frontal area and thereby decrease the pressure loss and reduce the structure weight, and on the other hand to decrease the heat capacity of the catalyst and enhance the temperature elevation speed of the catalyst. In this case, a large geometric surface area of the honeycomb structural body can be obtained so that it is also possible to realize a compact structure. However, the thin-walled ceramic honeycomb structure requires a careful handling because it is generally difficult to achieve a predetermined minimum guarantee value for the isostatic destruction strength as one index of the structural strength. Thus, damage of the catalyst carrier may be caused during installation operation, that is so-called "canning" for mounting the honeycomb structural body in a catalyst converter casing so as to prevent movement of the honeycomb structural body due to vibration and the like in a practical use condition. In this connection, a typical canning method is to hold the honeycomb structural body from the outer peripheral surface thereof, though there may be instances wherein the honeycomb structural body is held either solely in the flow passage direction, or from the outer peripheral surface and also in the flow passage direction. Therefore, it is generally considered necessary for the above-mentioned minimum guarantee value to be no less than 5 kgf/cm.sup.2, preferably no less than 10 kgf/cm.sup.2 in terms of isostatic destruction strength. Conventionally, reduction of the partition walls in the ceramic honeycomb structure and achievement of sufficient isostatic destruction strength have been recognized as problems of antinomy with each other, and there has not been known any ceramic honeycomb structure having a partition wall thickness of less than 0.170 mm, which can be put into practical use in a reliable and satisfactory manner.